1. Field of the Invention
This invention relates to a thermoplastic resin composition which is excellent in rigidity and impact resistance in respect of physical properties, has a short molding cycle in respect of injection moldability and is characterized in that its molded article is excellent in surface quality, for example, the molded article has neither flow mark nor weldline and is free from surface strain.
More particularly, it relates to a thermoplastic resin composition which comprises a major amount of a specific crystalline polypropylene and minor amounts of rubber components such as a specific ethylene-butene-1 copolymer rubber and the like and talc and which is excellent in rigidity and impact resistance in respect of physical properties, has a short molding cycle in respect of injection moldability and is excellent in surface quality of its molded article and to an injection-molded article excellent in dimension stability obtained by subjecting the thermoplastic resin composition to injection-molding, in particular, to an injection molded article, particularly for interior automotive trim (door trim, pillar or the like).
2. Prior Art
As a material for interior automotive trim, crystalline propylene-ethylene block copolymers have recently been used from the viewpoint of light weight, low cost and the like. However, conventional interior automotive trims made of a crystalline propylene-ethylene block copolymer are low in impact strength and contain a large amount of inorganic fillers for imparting thereto rigidity and thermal properties such as heat distortion temperature and the like. Therefore, the above interior automotive trims have a considerably large specific gravity.
It has been proposed in, for example, JP-A-53-22552 and JP-A-53-40045 to incorporate an ethylene-propylene copolymer rubber into a crystalline propylene-ethylene block copolymer for improving the impact strength of the latter. However, the crystalline propylene-ethylene block copolymer containing an ethylene-propylene copolymer rubber is inferior in rigidity and thermal properties such as heat distortion temperature and the like. For overcoming this disadvantage, it has been proposed in, for example, JP-A-51-136735, JP-A-53-64256, JP-A-53-64257, JP-A-57-55952, JP-A-57-207730, JP-A-58-17139, JP-A-58-111846, JP-A-59-98157 and JP-B-55-3374 and the like to incorporate an inorganic filler such as calcium carbonate, barium sulfate, mica, crystalline calcium silicate, talc or the like together with the ethylene-propylene copolymer rubber. Also, it is stated that by incorporating talc in JP-A-51-1236735 and by incorporating talc, mica or calcium silicate in JP-A-57-07630, the molding shrinkage can be made smaller and the dimension stability can be improved.
Also, JP-A-58-17139 and JP-A-58-17140 propose to incorporate into the crystalline propylene-ethylene block copolymer an ethylene-butene-1 copolymer rubber in place of the ethylene-propylene copolymer rubber. In particular, JP-A-58-17140, it is stated that as compared with the ethylene-propylene copolymer rubber, the use of an ethylene-butene-1 copolymer rubber can make the impact whitened area small and can improve the resistance to wounding.
The crystalline propylene-ethylene copolymer/ethylene-propylene copolymer rubber/talc composition (referred to hereinafter as the ethylene-propylene copolymer rubber based composition) has been widely used as a material for interior automotive trims and is molded by a conventional injection molding method into interior automotive trims because the ethylene-propylene copolymer rubber based composition is inexpensive and has good moldability. In respect of injection moldability, it is required that firstly the molding cycle of the composition can be shortened to increase the productivity and secondly, when the above composition is subjected to injection molding, there can be given an molded article for interior trim which has such an excellent surface quality that the molded article has neither flow mark nor weldline and is free from surface strain.
However, the conventional ethylene-propylene copolymer rubber based composition can be improved in surface quality in the injection molding by simply enhancing the flow properties; however, the molded article obtained cannot satisfy the impact strength required for the interior trim. On the other hand, though the filling time can be shortened, the plasticizing time becomes longer, so that it has a problem that the molding cycle time is consequently not shortened.